This invention relates to a polymer electrolyte membrane suitable for use in fuel cells. The invention also relates to a polymer electrolyte membrane suitable for use in fuel cells that has not only outstanding oxidation resistance, heat resistance and dimensional stability but also outstanding electrical conductivity.
Fuel cells using polymer electrolyte membranes feature high energy density, so using methanol, hydrogen and the like for fuel, they hold promise for use as power supplies or convenient auxiliary power supplies to mobile communication devices, household cogeneration systems and automobiles. One of the most critical aspects of the fuel cell technology is the development of polymer electrolyte membranes having outstanding characteristics.
In a polymer electrolyte fuel cell, the electrolyte membrane serves to conduct protons and it also plays the part of diaphragm which prevents direct mixing of the fuel hydrogen or methanol with the oxidant air (oxygen). The electrolyte membrane has several requirements to meet: large ion-exchange capacity; sufficient chemical stability of the membrane to allow for prolonged application of an electric current, in particular, high resistance (oxidation resistance) to hydroxide radicals and the like which are principal factors that contribute to deterioration of the membrane; heat resistance at 80° C. which is the cell operating temperature and above; and constant and high water retention of the membrane which enables it to keep low electrical resistance. In addition, the electrolyte membrane which also plays the part of diaphragm is required to have outstanding mechanical strength and dimensional stability, as well as having no excessive permeability to hydrogen gas, methanol or oxygen gas.
Early models of the polymer electrolyte fuel cell used a hydrocarbon-based polymer electrolyte membrane produced by copolymerization of styrene and divinylbenzene. However, this electrolyte membrane, being very poor in durability due to low oxidation resistance, was not highly feasible and later extensively replaced by perfluorosulfonic acid-based membranes such as DuPont's Nafion®.
The conventional fluorine-containing polymer electrolyte membranes such as Nafion® have outstanding chemical durability and stability but, on the other hand, their ion-exchange capacity is very small (ca. 1 meq/g) and, due to insufficient water retention, the ion-exchange membrane will dry up, thereby impedes proton conduction or, in the case of using methanol for fuel, causes swelling of the membrane or cross-over of the methanol.
If, in order to increase the ion-exchange capacity, one attempts to introduce more sulfonic acid groups, the membrane which has no cross-linked structure in polymer chains will swell and its strength drops markedly, whereby it will break easily. Therefore, with the conventional fluorine-containing polymer electrode membranes, the quantity of sulfonic acid groups had to be adjusted to small enough levels to guarantee the required membrane strength, so that one could only produce membranes having ion-exchange capacities of no more than about 1 meq/g.
In addition, the fluorine-containing polymer electrolyte membranes such as Nafion® have the problem of difficult and complex monomer synthesis; what is more, the step of polymerizing the synthesized monomers to produce the intended polymer membrane is also complex and yields a very expensive product, thereby presents a large obstacle to realizing a commercial proton-exchange membrane fuel cell that can be installed on automobiles and other equipment. Hence, efforts have been made to develop low-cost and high-performance electrolyte membranes that can be substituted for Nafion® and other conventional fluorine-containing polymer electrolyte membranes.
In the field of radiation-induced graft polymerization which is closely related to the present invention, attempts are being made to prepare solid polymer electrolyte membranes by grafting onto polymer membranes those monomers into which sulfonic acid groups can be introduced. The present inventors conducted intensive studies with a view to developing such new solid polymer electrolyte membranes and found that solid polymer electrolyte membranes characterized by a wide range of controllability of ion-exchange capacity could be produced by first introducing a styrene monomer into a poly(tetrafluoroethylene) film having a cross-linked structure by radiation-induced graft reaction and then sulfonating the grafts. The solid polymer electrolyte membrane having such characteristics and the process for producing it were applied for patent (JP 2001-348439 A). However, the styrene graft chains in this polymer electrolyte membrane were composed of hydrocarbons, so when an electric current was passed through the membrane for a prolonged period of time, the graft chains were partly oxidized to lower the ion-exchange capacity of the membrane.
The present inventors also found that solid polymer electrolyte membranes characterized by a wide range of ion-exchange capacity and outstanding oxidation resistance could be produced by first performing radiation-induced grafting or co-grafting of a fluorine-containing monomer on a poly(tetrafluoroethylene) film having a cross-linked structure and then introducing sulfone groups into the graft chains. The solid polymer electrolyte membrane having such characteristics and the process for producing it were applied for patent (JP 2002-348389 A). However, as it turned out, the ordinary fluorine-containing polymer membranes had the problem that the graft reaction of the fluorine-containing monomer was difficult to get deep into the membrane and that depending on the reaction conditions, the graft reaction was restricted to the film surface, thus making it difficult to improve the characteristics of the film as an electrolyte membrane.
The present invention was accomplished in order to solve the aforementioned problems of the prior art and has as an object providing a solid polymer electrolyte that is free from the defects of the conventional polymer ion-exchange membranes, namely, small ion-exchange capacity, low dimensional stability of the membrane and, in particular, low oxidation resistance which is the most critical disadvantage.